In recent years, lithium-ion secondary batteries have come into practical use as electricity storage devices having high energy density.
Conventionally, carbon-type materials are generally used as the negative electrode material capable of occluding and releasing lithium ions in a lithium-ion secondary battery. However, in a negative electrode for which a carbon material is used, the reaction potential at lithium ion occlusion or release is as low as 0.1 V (vs. Li/Li+) or less, which is close to the dissolution/deposition potential of lithium. Therefore, deposition of metal lithium on the negative electrode surface is likely to occur particularly during rapid charging in a low temperature environment, in which reactivity decreases. Deposition of metal lithium on the negative electrode not only causes internal short-circuiting, but also induces decrease in capacity due to passivation, and may possibly cause operation insufficiencies and performance deteriorations of the battery.
Accordingly, oxide-type materials such as lithium titanate are being studied as alternative negative electrode materials to carbon materials. Lithium titanate is a complex oxide having a spinel-type crystal structure similar to that of lithium manganate, which is used as a positive electrode material, and thus is capable of reversibly occluding and releasing lithium ions. Unlike lithium manganate, lithium titanate reversibly reacts with lithium at a potential as low as about 1.5 V (vs. Li/Li+), and therefore can be used the negative electrode material. Moreover, it undergoes very little volumetric change in charging and discharging reactions at this potential, and is expected to have excellent characteristics as an electrode material for secondary batteries to undergo repetitive charging and discharging.
As an example of a known lithium titanate, Patent Document 1 discloses a negative-electrode active material of the composition expressed as LiaTi3−aO4 (where a in the formula represents a number such that 0<a<3), in which primary particles with an average particle size of less than 1 μm constitute secondary particles with an average particle size of 5 to 100 μm.
On the other hand, Patent Document 2 discloses a lithium titanate with an average pore diameter of 5 nm to 50 nm and a pH ranging from 10 to 11.2. Patent Documents 1 and 2 disclose that a secondary battery with excellent large current characteristics and cycle characteristics can be realized.